Transcript F10: Fault-Tolerant Engineered Networks

Full-duplex Backscatter for <1 μW
Vincent Liu
Vamsi Talla, Shyam Gollakota
Why Is This Hard?
Existing solutions require O(1W)
1,000,000x too much!
Full-duplex Backscatter
First full-duplex design for battery-free devices
Same frequency, single antenna
Full-duplex Backscatter
RF source
Data (1 kbps)
Alice
Feedback (100 bps)
Bob
Brings self-interference down to the noise floor
using < .75 uW
Capacitor Charge
Why Do We Care?
Energy Is The Bottleneck!
Threshold to
transmit a packet
Recovery from collisions and errors
takes a long time
Time
• Capacitor charging is exponential in time
Why Do We Care?
Energy Is The Bottleneck!
• Full-duplex can help fix this problem
• Instantaneous feedback enables a better
network
– Collision detection (100x recharge time reduction)
– Error correction (10x overhead reduction for every packet)
• How do we get full-duplex on battery-free devices?
• How do we use instantaneous feedback to design a
power-efficient network stack?
• How do we get full-duplex on battery-free devices?
• How do we use instantaneous feedback to design a
power-efficient network stack?
Backscatter Overview
Alice absorbs:
Existing signal
RF Source
TV Tower
(RF source)
Bob
Alice
Signal at Bob
Ambient
Backscatter
Bob (Receiver)
Alice reflects:
Existing signal
& Reflection
Additional
Multipath
Alice (Sender)
Time
TV
(Legacy receiver)
Challenge: Bob’s Reflections Lose Information
RF Source
Alice absorbs:
Existing signal
TV Tower
(RF source)
Bob
Alice
Signal at Bob
Ambient
Backscatter
Additional
Multipath
Bob absorbs
Bob (Receiver)
Alice reflects:
Existing signal
& Reflection
Bob reflects
Alice (Sender)
Time
TV
(Legacy receiver)
Solution: Change Phase Instead
Bob sends ‘0’:
Existing signal
& θ shift
RF Source
TV Tower
(RF source)
Bob sends ‘1’:
Existing signal Alice
& -θ shift
Alice can decode
Ambient
Bob’s signal
Bob
Bob can decode
Additional
Alice’s signal
Signal at Bob
Backscatter
Multipath
Bob absorbs
Bob (Receiver)
Bob reflects
Alice (Sender)
Time
TV
(Legacy receiver)
Practically, phase modulation is imperfect
Residual interference
Solution: Leverage Difference in Rates to
Eliminate Residual Interference
Data (1 kbps)
Alice
Feedback (100 bps)
Remove interference with
high-pass filter
Bob
Remove interference with
low-pass filter
Can be done with cheap, passive circuits at
NO additional power
Is Our Cancellation Effective?
10
-10
Magnitude (dB)
-30
Noise floor
-50
Original
-70
With phase
modulation
Total
cancellation
-90
-110
-130
-150
0
200
400
600
Frequency (Hz)
800
1000
Reduces self-interference to noise floor
• How do we get full-duplex on battery-free devices?
• How do we use instantaneous feedback to design a
power-efficient network stack?
Challenge: Energy Is the Bottleneck
• Recovery from collisions/errors takes a long time
• We use Full-duplex Backscatter to achieve
instantaneous feedback
Protocol: Acknowledge Data at a Bit Level
Forward data
channel
Feedback
channel
40 bits
1
2
3
1
4
2
5
3
4
4 bits
Time
1. Split packet into 40-bit chunks
2. Receiver returns a checksum of each chunk
3. Sender verifies checksum
5
Does Full-duplex Help With Collisions?
Detect collisions and stop transmissions
1000
Full-duplex Backscatter
Conventional
Recharge Time (ms)
100
10
1
0.1
0.01
-6
100x
-4
-2
0
improvement
in recharge
Input Power
(dBm)
64-byte packet size
2
time
4
Does Full-duplex Help With Error Correction?
Recover from errors by retransmitting only the failed bits
10000
Full-duplex Backscatter
Conventional
Average Overhead (%)
1000
100
10
1
0.1
0
0.001
>90%
0.002
0.003
0.004
decrease
inRate
overhead
Bit Error
64-byte packet size
0.005
Conclusion
• We show
– The first full-duplex design for battery-free devices
– A power-efficient network stack that has significant
gains across the board
• Re-design networking primitives with power as a
first-class citizen
– MIMO (SIGCOMM’14), coding (SIGCOMM’14), UWB
(?), TCP/IP (?), …